A transmitter employs beamforming in order to amplify transmitted signals in selected directions, while weakening them in others. To beamform, the transmitter transmits a signal from multiple transmit antennas, but with individually adjusted phase shifts or time delays. This effectively creates a beam in the resulting transmit radiation pattern of the signal—e.g., through controlled constructive and destructive interference of the phase-shifted signals from individual antenna elements. The beam direction depends on the phase shifts of the antenna elements. Correspondingly, a receiver employs beamforming in order to amplify signals received from selected directions while weakening unwanted signals in other directions. The receiver does so by using phase shifts between antenna elements to steer the maximal antenna sensitivity toward a desired direction.
To support base station beamforming, a base station in some approaches deploys multiple beams that fixedly point in respective directions. When the base station transmits data to a user equipment (UE) on one of those beams, the base station constantly evaluates the quality of the data transmission. When the quality degrades to the point of being below an unacceptable threshold, the base station triggers a process to switch the data transmission onto a different one of the beams.
Towards this end, the base station identifies a set of beams as being candidates for switching the data transmission onto. The base station identifies this set as including those beams (from the base station or any neighboring base station) which provide coverage in the vicinity of the UE's last known position. An activation table at the base station may for instance map UE position to relevant candidate beams. The base station then instructs the UE to measure reference signals that are respectively transmitted on the candidate beams in the set. The base station uses the measurement results to decide which beam to switch the data transmission onto. This conventional beam tracking approach thereby switches between different beams “reactively”, responsive to quality degradation and based on a UE's current position.